A description of a typical human walking cycle (i.e., gait) begins with a heel strike to the ground, followed by a mid-stance phase in which the front of the foot lowers to the ground, pivoting about the grounded heel. The gait then transitions to a toe-off phase, in which the heel is lifted with an associated forward motion of the leg and body on the ball and toes of the foot. Ultimately, the foot is completely lifted from the ground and swung forward in a swing-through phase to the next heel strike. The other foot undertakes the same cycle of motion in a generally coordinated manner to provide forward locomotion.
During this complex motion, each knee transitions from a relatively straight extension at heel strike to a rearward bend, or flexion, through the toe-off phase, and returns to extension during the final swing-through phase. During the cycle, the weight of the patient is borne through the knee to varying degrees. The ankle has similar transitions, first dampening ground reaction forces with toe-off, then re-directing ground reaction force during forward progression, plantarflexion and push-off, and finally dorsiflexion to return the ankle to neutral and assist foot clearance during swing.
The human knee and ankle systems can suffer a number of pathologies that affect the patient's ability to bear this weight and walk (with or without pain). Orthotic ankle and knee devices are primarily directed to supporting and stabilizing the joint in response to muscle weakness and/or joint instability. The devices support, guide, and limit the range of motion of the knee joint during the gait cycle. However, traditional orthotic devices are prone to rigidity in movement, and do not provide flexion and extension capabilities approximating that of a healthy, normal joint. For instance, during a normal walking motion, there exists a certain degree of muscle resistance during ankle or knee flexion, and a certain degree of shock absorption by the quadriceps, for instance, upon heel strike, thereby causing knee flexion and preventing the impact from permeating up the leg to the hips and back, as can occur with a stiff-legged, strutting gait.
For the foregoing reasons, it is an objective of orthotic devices to provide fundamental support, while additionally providing versatility of motion that, to the greatest extent possible, resembles normal joint and muscle function to absorb ground reaction forces and redirect them toward forward progression. Further, an orthotic device closely approximating normal joint and muscle motion can help prevent a learned disuse of certain muscles and movements during a period of prolonged immobility and/or rehabilitation, whereby the brain settles on compensatory muscle use and movements that greatly inhibit mobility, eventually requiring heavier and more restrictive devices resulting in more noticeable limp and an inefficient gait.